A large centeralized hold of the substance being sold. The 'main supply' -brick\kilo etc- form which smaller amounts are cut, portioned down and sold off. What makes it 'da bomb'? Depends on the scale of the selling activity and the substance in question. This usage goes back to the early 90s. Most narcotics cops use this term too in certain metro areas (chicago, Miami, others) when asking lower end pushers who just got busted "Who has da bomb?" and "Where is da bomb?" also "who is holding da bomb?

Guy1: How'd you get so much?
Guy2: <dealers name> had da bomb.
Guy1: Where?
Guy2: I went to his motel room. He had da bomb. Just arrived.
Guy1: Yeah?
Guy2: So it worked out like that.